Precision press molding is a known method of molding optical products such as lenses to high precision shapes and dimensions with good production properties. When the temperature used to mold the glass employed in precision presses reaches high temperature of 600° C. or more, there are problems in that the surface of the pressing mold is damaged and the durability of the mold material decreases. Thus, the transition temperature of the glass must be lower than 600° C. However, commercially available optical glass, with optical characteristics of a refractive index (nd) of around 1.8 and an Abbé number (νd) of about 30 to 45, has a high glass transition temperature (Tg) of 600° C. or more and is thus not suited to precision press molding.
To solve this problem, glass incorporating a large amount of an alkali such as Li2O has been proposed. One example is the SiO2—B2O3—La2O3—Ta2O5—ZnO—Li2O glass described in Japanese Unexamined Patent Publication (KOKAI) Heissi No. 6-305769. The glass described in the above-mentioned publication incorporates a large quantity of Ta2O5 as an essential component for reducing the glass transition temperature. However, Ta2O5, that is basically a scarce material to begin with, has in recent years been in great demand as a capacitor material causing the cost to skyrocket. Thus, it has become difficult to provide glass employing large amounts of Ta2O5 at low cost, or to produce such glass in large amounts.
The present invention was devised to solve the above-stated problems. It is an object of the present invention to provide optical glass having a refractive index (nd) of 1.75 to 1.87, an Abbé number (νd) of 30 to 45, and excellent low-temperature softening properties even when not incorporating Ta2O5 as well as permitting a low production cost; a precision press molding preform and an optical element comprised of such glass; a method of manufacturing the preform; and a method of manufacturing the optical element.